KR940003931Y1 - Image receiving circuit for tv - Google Patents

Abstract

No content.

Description

Video receiver

1 is a circuit diagram showing an embodiment of the present invention.

2 is a circuit diagram showing a conventional AFT circuit.

3A and 3B are circuit diagrams showing an example of a specific circuit of a low pass filter.

4 is a characteristic diagram of the lowpass filter of FIG.

5 is a circuit diagram showing another example of the configuration of main parts of the conventional AFT circuit.

* Explanation of symbols for main parts of the drawings

3: mixed circuit 5,6: IF amplifier circuit

7: limiter circuit 8: AFT detection circuit

11: local oscillator circuit 16: low pass filter

The present invention relates to an AFT (automatic fine tuning) circuit of an image receiving circuit used in an imaging device such as a television receiver, and more particularly, to an image receiving circuit that removes unnecessary harmonics generated by a limiter circuit in an AFT circuit. It is about.

In television receivers and the like, an AFT circuit for adjusting the oscillation frequency of a local oscillation circuit has been conventionally used, as shown in Japanese Patent Laid-Open No. 60-194,609. FIG. 2 shows such an AFT circuit, in which the video RF signal from the antenna 1 is amplified by the RF amplifier circuit 2 and converted into a video IF signal by the mixing circuit 3. The image IF signal is amplified by the first IF amplifier circuit 5 through a surface acoustic wave (SAW) filter 4 and then detected by the image detector circuit 50. On the other hand, the amplified video IF signal is amplified again by the second IF amplifier circuit 6, and then amplitude-limited by the limiter circuit 7, and FM-detected by the AFT detection circuit 8. Therefore, in the AFT detection circuit 8, a normal voltage according to the frequency of the video IF signal is issued, amplified by the DC amplification circuit 9, and then, to the local oscillation circuit 11 through the tuning control circuit 10. The oscillation frequency of the local oscillation circuit 11 is applied.

Therefore, according to the AFT circuit of FIG. 2, the frequency of the output video IF signal of the mixing circuit 3 can be kept constant at all times despite temperature change or time change. In FIG. 2, the dashed-dotted line indicates the integrated circuit 12. As shown in FIG.

Incidentally, in the AFT circuit of FIG. 2, the limiter circuit 7 is arranged to perform FM detection, but there is a problem that harmonics of the video IF signal are generated when amplitude limiting is performed in the limiter circuit 7. In general, in order to prevent the influence of the harmonics on the local oscillation circuit 11, a noise canceling circuit is provided between the integrated circuit 12, the terminal 13, and the local oscillation circuit 11. By the way, the harmonic is an antenna for the terminal 15 to which the abnormality coil 14 of the terminal 13 or the AFT detection circuit 8 is connected, the aluminum wiring of the integrated circuit 12, or the like. It is radiated to outside. At this time, if the integrated circuit 12 is arranged near the tuner, there is a problem that the harmonics jump into the tuner's RF input on the feeder line from the antenna 1. For example, in Japan, the frequency of the video IF signal is 58.75 MHz, but three times the harmonic frequency thereof is 176.25 MHz. In addition, the four-channel frequency band of Japan's VHF is 170-176 MHz, and is very close to the frequency of the harmonics. Therefore, when receiving the broadcast of 4 channels, there was a problem that streaks due to frequency bits appeared on the television screen due to interference caused by the above harmonics. In addition, in the United States, since the IF frequency is 45.75 MHz, four times the harmonic 183 MHz affects the eight-channel frequency band (181.25 MHz-187.25 MHz).

In this situation, however, the AFT detection circuit 8 in the IC 12 is a so-called balanced input type (in this case, as shown in FIG. 5, the abnormal coil 14 is connected to the AFT detection circuit 8). Connected between a pair of terminals 15 (15 ')], so that harmonic signals are attenuated by the so-called in-phase cancellation action of the differential pair transistors in the AFT detection circuit 8, which is not a problem. .

However, as shown in FIG. 2, when the AFT detection circuit 8 is configured in an unbalanced state (in this case, the abnormal coil 14 is connected to one terminal 15), the in-phase cancellation operation described above is performed. Because harmonics are not attenuated by harmonics, harmonics leak out through the wiring in the IC 12, which is a big problem.

The present invention has been made in view of the foregoing, and includes a mixed circuit for converting a video RF signal into a video IF signal, a video IF amplifier circuit for amplifying the video IF signal from the mixed circuit, and a video IF amplifier circuit. An image having a limiter circuit for limiting the amplitude of the IF signal from the image signal, an AFT detection circuit for FM detection of the output signal of the limiter circuit, and a local oscillation circuit whose oscillation frequency changes according to the detection output of the AFT detection circuit. A receiving circuit, characterized in that a low pass filter is provided between the limiter circuit and the AFT detection circuit.

According to the present invention, since a low pass filter for removing harmonics is provided immediately after the limiter circuit in which harmonics are generated, the adverse effect of the harmonics on the rear end circuit can be completely prevented.

1 is a circuit diagram showing an embodiment of the present invention, wherein 16 is a low pass filter connected between the limiter circuit 7 and the AFT detection circuit 8.

In addition, in FIG. 1, the same code | symbol is attached | subjected about the circuit element similar to FIG. 2, and the description is abbreviate | omitted.

Next, the operation will be described. The image RF signal received at the antenna 1 is amplified by the RF amplification circuit 2, frequency-converted from the mixing circuit 3 to the image IF circuit, and the first in the IC 12 through the SW filter 4. And second IF amplifier circuits 5 and 6, in turn. In addition, the video IF signal from the first IF amplifier circuit 5 is detected by the video detection circuit 50. The video IF signal sufficiently amplified by the first and second IF amplifier circuits 5 and 6 is input to the low pass filter 16 after amplitude limitation in the limiter circuit 7. The amplitude limited video IF signal includes many harmonics as described above, but by setting the cutoff frequency of the lowpass filter 16 to an appropriate value, all the harmonics can be cut off. Specific circuit examples of the low pass filter 16 are shown in FIGS. 3a and b. FIG. 3A is a circuit diagram showing a two stage passive low pass filter, the filter characteristic of which is the same as the solid line A in FIG. As is apparent from the solid line A, the lowpass filter of FIG. 3A allows the image IF signal (58.75 MHz) to pass and the triplex harmonic component (176.25 MHz) to be attenuated. FIG. 3B is a circuit diagram showing a low pass filter in which the low pass filter of FIG. 3A is formed in an active type, and the filter characteristics thereof are the same as the solid line B in FIG.

The solid line B can reduce the level reduction of the video IF signal lower than the solid line A. The dotted line in the high range of the solid line B is a case where the time constants of the resistor 16 and the capacitor 17 are set small, for example in FIG. 3B, and the peaking (high frequency emphasis) characteristic is maintained in the low pass filter. Display.

In this way, the pass rate of the video IF signal can be improved again, and the filter characteristics can be made good.

In this way, since the video IF signal containing no harmonic components is input to the AFT detection circuit 8, a DC voltage corresponding thereto is generated at the output terminal thereof. At that time, harmonic components are not undesirably radiated from the abnormal coil 14 connected to the AFT detection circuit 8. As a result, a situation in which harmonics enter the IF amplifier circuit 2 is prevented. The DC voltage from the AFT detection circuit 8 is amplified by the DC amplifier circuit 9 and applied to the local oscillation circuit 11 to control the local oscillation frequency. Therefore, an image IF signal having a constant frequency can be obtained at the output terminal of the mixing circuit 3 at all times.

Therefore, when the video receiving circuit according to the present invention is used for a video receiver for receiving a television signal, for example, a television receiver, a video tape recorder, or the like, the reception can be performed in the most suitable state.

As described above, according to the present invention, it is possible to prevent mixing of harmonics generated in the limiter circuit in the AFT circuit into the RF stage, thereby preventing streaks appearing on the television screen. In addition, according to the present invention, since the local oscillation circuit is not affected by the harmonics, the image IF signal generated at the output terminal of the mixed circuit can always keep the frequency constant despite temperature change or time change.

Claims (5)

A mixed circuit for converting a video RF signal into a video IF signal, a video IF amplifier circuit for amplifying the video IF signal from the mixed circuit, and a limiter circuit for limiting the amplitude of the video IF signal from the video IF amplifier circuit. And an AFT detection circuit for FM-detecting the output signal of the limiter circuit, and an image receiving circuit including an oscillation circuit in accordance with the detection output of the AFT detection circuit, wherein a low pass is passed between the limiter circuit and the AFT detection circuit. An image receiving circuit comprising a filter. The video receiving circuit according to claim 1, wherein the low pass filter is an active low pass filter. The video receiving circuit according to claim 1, wherein the cutoff frequency of the low pass filter is set between the video IF signal frequency and a frequency three times the frequency. 2. An image receiving circuit according to claim 1, wherein an image IF amplifier circuit, a limiter circuit, an AFT detection circuit, and a low pass filter are incorporated in a single integrated circuit. 3. An image receiving circuit according to claim 2, wherein the peaking characteristic is maintained in an active low pass filter.